Today, it is a very common to apply onto a ship's bottom and the like an antifouling coating composition comprising an organic or inorganic antifouling agent and a resinous binder, such as a vinyl resin, alkyd resin and the like.
In that case, since the antifouling effect is fully dependent on the antifouling agent dissolved out of the coating and the dissolution of said agent is primarily a diffusion phenomenon caused by a concentration gradient of said agent in the coating, one cannot expect a long lasting, stable antifouling effect with them.
Furthermore, since the water insoluble resinous component will, after dissolution of said agent from the coating, form a skeleton structure, there are additional problems as increase in resistance of friction between the ship surface and water, lowering of sailing speed, increase in sailing fuel and the like. Under the circumstances, an antifouling coating composition comprising an antifouling agent and a hydrolyzable resin vehicle capable of forming a comparatively tough coating and being gradually decomposed by hydrolysis in sea water has become the center of public attention.
The present inventors had already found that a class of polyester resins having a number of metal-ester bonds in their polyester backbone chains are useful as a resinous vehicle in a polishing type antifouling paint, and applied for patent as Japanese Patent Application Nos. 165922/81 and 196900/83.
Such resins are easily hydrolyzed, under weak alkaline condition as in sea water, at the metal-ester bonding portions, are disintegrated into a number of small, low molecular weight segments and are dissolved in sea water. However, said resins are primarily of comparatively low molecular weight (e.g. upto about 2000) and are poor in film-forming property, and therefore, said coating compositions still have the problems of easy occurence of cracks and peeling of the formed coatings.
If the molecular weight of said polyester resin is increased to a moderate level, it is indeed possible to improve the film-forming property, but, at that time, it will necessarily be attended with a marked decrease in hydrolysis property of the resin. To compensate the same, if the metal-ester bonding in the backbone chain of the resin is increased in number, this will cause additional problems that the resultant resin is only soluble in a polar solvent and not in most solvents commonly used in a coating composition, and that the formed coating is swollen with sea water. These attempts would therefore, give no fruitful results, and thus, there leaves much to be desired. An attempt has also been made to use a resin whose side chain has a trialkyl tin ester portion as a terminal group. In this type of resin, polarity of the resin is gradually increased in proportion to the progress in hydrolysis of said ester portion, and the resin is finally dissolved in sea water.
Typical examples of such resins are acrylic resins having as a constitutional element triorganotin salts of .alpha.,.beta.-unsaturated basic acids. In this case, in order to obtain a stabilized, tough coating, the resin should preferably be free from hydrophilic groups if circumstances allow, and in order to ensure the dissolution of the hydrolyzed resin in sea water, the resin should preferably have as many hydrophilic groups as possible, i.e. more than a certain critical range, after said hydrolysis.
Therefore, in the preparation of such resin by the copolymerization of triorganotin salt of .alpha.,.beta.-unsaturated basic acid and other acrylic vinyl monomers, attempts have been made such that the former is presented in a higher concentration in the reaction system and the latter is selected from members with no or the least amount of hydrophilic groups. Thus, a copolymer of acrylate, acrylamide, styrene and the like containing 55 to 70 wt % of triorganotin salt of .alpha.,.beta.-unsaturated monobasic acid has been prepared and practically used.
In this type of resin, differing from the aforesaid polyester resin having metal-ester bondings in its main chain, hydrophilic carboxyl groups are generated at the time when the triorganotin portions at the side chains are released through hydrolysis and the resin is only dissolved in sea water at the stage where the concentration of said carboxyl groups get to a certain critical point. The film-forming property of the resin is also excellent. However, there is a problem in that a considerable quantity of highly expensive and toxic organotin compound is essential. Therefore, from both hygienic and economic point of view, it has long been desired to obviate the use of such material.